The present invention relates to product checkout devices and more specifically to a produce data collector including an enhanced linear variable filter (LVF) spectrometer.
Bar code readers are well known for their usefulness in retail checkout and inventory control. Bar code readers are capable of identifying and recording most items during a typical transaction since most items are labeled with bar codes.
Items which are typically not identified and recorded by a bar code reader are produce items, since produce items are typically not labeled with bar codes. Bar code readers may include a scale for weighing produce items to assist in determining the price of such items. But identification of produce items is still a task for the checkout operator, who must identify a produce item and then manually enter an item identification code. Operator identification methods are slow and inefficient because they typically involve a visual comparison of a produce item with pictures of produce items, or a lookup of text in table. Operator identification methods are also prone to error, on the order of fifteen percent.
A produce data collector which uses an active spectrometer is disclosed in the cited co-pending application. A produce item is placed over a window in the produce data collector, the produce item is illuminated, and the spectrum of the diffuse reflected light from the produce item is measured.
The spectrometer may include an LVF, which offers continuous spectral coverage within the visible wavelength range (400-700 nm). The LVF consists of a rectangular glass substrate and a filter coating on the glass substrate. The LVF is formed by vacuum depositing dielectric coating materials as multi-layer thin films onto the substrate. The thickness of the coating is uniform along one direction (small side or end) of the glass substrate but varies continuously along the perpendicular or wavelength direction (long side). Portions of the LVF representing desired wavelengths or wavelength ranges may be used for specific applications. In the produce data collector disclosed in the co-pending application, the LVF is placed on top of a linear detector array.
There are two important characterizations of the LVF, its resolution in terms of full-width-at-half-maximum (FWHM) and its spectral density. The spectral density measures how much the center wavelength changes per unit distance along the length of the LVF. A typical LVF, such as those manufactured by Optical Coating Laboratory, Inc., the FWHM is less than 2.5 percent of the wavelength and there are different spectral density values available, typically in the range of 30 to 60 nanometers per millimeter.
Due to its special manufacturing process, LVF is usually expensive. It is highly desirable to use portions of an LVF in applications. This causes a serious difficulty, the light scattered off the interior side walls of the glass substrate and the LVF housing generate serious optical noises that deteriorate the performance of the spectrometer. The problem exists as long as the whole top surface area of the LVF is illuminated, even if all incident rays are limited to within a very narrow acceptance cone by using an effective baffle system.
Since the interior side walls of the LVF substrate are in general not optically flat, an incident ray hitting these walls will be scattered and some of these scattered rays will pass through the filter coating with larger incident angles, which deteriorates the spectral performance of the filter in two ways, it reduces the resolution and it generates background noise in the spectrum.
Light outside the operation range of the LVF is also a problem. For example, infrared energy poses a particular problem. Infrared signal outside the wavelength range of the LVF will pass through at the corresponding half-wavelength band. For example, infrared signal at 800 nm will pass through the 400 nm band. Such infrared xe2x80x9cleakagexe2x80x9d may be treated as another type of noise and can be quite significant for many applications.
All these noise problems become very important when one tries to manufacture LVF/LDA spectrometers to narrow tolerances and high quality standards.
Therefore, it would be desirable to provide an LVF spectrometer which minimizes or eliminates the noise problems addressed above. It would also be desirable to provide a produce data collector which includes such an LVF spectrometer.
In accordance with the teachings of the present invention, a produce data collector including an enhanced linear variable filter (LVF) spectrometer is provided.
The produce data collector includes a light source for illuminating a produce item, a spectrometer to obtain spectral information about the produce item in incoming reflected light from the produce item, and control circuitry which produces digital signals from electrical signals produced by the spectrometer.
The spectrometer includes a linear variable filter which includes a primary surface and a number of interior surfaces and which splits incoming light reaching the primary surface into a number of light portions having different wavelengths, a photodetector adjacent the linear variable filter which samples the light portions and produces electrical signals containing information about each light portion, and an optical slit member above the primary surface of the linear variable filter which has a slit with a width sufficient to minimize scattering of the incoming light by the interior surfaces of the linear variable filter.
The optical slit member preferably also minimizes scattering of the incoming light by interior walls of a housing containing the LVF.
The spectrometer preferably also includes a filter which minimizes light within the incoming light which is outside a wavelength range of operation of the linear variable filter.
It is accordingly an object of the present invention to provide a produce data collector including an enhanced LVF spectrometer.
It is another object of the present invention to provide an LVF spectrometer which minimizes noise from scattered light.
It is another object of the present invention to provide an LVF spectrometer which minimizes noise from leakage of wavelengths, such as infrared wavelengths, outside the operating range of the LVF.
It is another object of the present invention to provide a produce data collector which uses an LVF spectrometer which is resistant to noise from scattered light, the FOV effect, and leakage.